1. Field of the Invention
This invention relates to the production of 1-butene, and, in particular, the production of high purity 1-butene.
2. Description of the Prior Art
Heretofore, high purity 1-butene has traditionally been produced from a mixture of hydrocarbons having 4 carbon atoms per molecule (C4s). The desired 1-butene was distilled from this C4 mixture.
This C4 mixture was obtained from a plurality of varying processes. One exemplary process is the thermal cracking of a hydrocarbon stream in an olefin production plant. Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In an olefin production plant, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep hydrocarbon molecules separated. This mixture, after preheating, is subjected to hydrocarbon thermal cracking at elevated temperatures of from about 1,450 degrees Fahrenheit (F or ° F.) to about 1,550F in a pyrolysis furnace (steam cracker).
The cracked product from the pyrolysis furnace contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, inclusive). This product contains aliphatics, aromatics, saturates, and unsaturates, and can contain significant amounts of molecular hydrogen (hydrogen).
This cracked product is then further processed in the olefin plant to produce, as products of the plant, various separate streams such as hydrogen, ethylene, propylene, pyrolysis gasoline, and a crude C4 stream. This C4 stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, butadienes (cis and trans isomers), and hydrogen.
Heretofore, this crude C4 stream has been subjected to hydrogenation to remove acetylenes, extractive distillation to remove diolefins such as butadiene isomers, and an etherification step to convert at least some of the isobutylene to methyl t-butyl ether. The methyl t-butyl ether is separated as a product of the process thereby leaving a separate raffinate product that contains 1-butene, 2-butenes, isobutane, n-butane and, at most, only traces of butadienes and isobutylene. As disclosed in detail later, the raffinate components other than 1-butene are all close boiling to 1-butene, and 1-butene is in the middle of the relevant boiling range. For example, isobutane is lighter (boils at a lower temperature) than 1-butene, whereas n-butane and 2-butenes are heavier (boil at a higher temperature) than 1-butene. Also 1,3 butadiene and isobutylene boil especially close to 1-butene, and, therefore, are troublesome even in trace amounts when the goal is high purity 1-butene. This is recognized in U.S. Pat. No. 5,698,760 to Kelly which is discussed in detail hereinafter.
Also heretofore, this raffinate product has been subjected to metathesis of its 2-butene content with ethylene to form propylene. After the separate removal of ethylene for recycle to the metathesis zone and propylene as a product of the process, the resulting stream contains a substantial amount of 2-pentenes (80 to 90 weight percent (wt %) based on the total weight of the stream), the remainder being a mixture of 1-pentene, 1-butene, 2-butenes, and trace amounts of hydrocarbons having 6 carbon atoms per molecule (C6s). A mixture of butenes, n-butane, and isobutane has been removed from this stream and sent on to alkylation or thermal cracking due to its n-butane and isobutane content, leaving a separate mixture of 2-pentenes (cis and trans isomers) in the amount of at least about 80 wt % based on the total weight of the separate mixture, up to about 15 wt % 1-pentene, and up to about 5 wt % C6s. This invention can employ as its feed material this mixture of 2-pentenes, 1-pentene, and C6s. This mixture does not contain 2-methylbutene-2 which is required by the Kelly patent cited above. As disclosed in greater detail hereinafter, other streams containing hydrocarbons having 5 carbon atoms per molecule (C5s), including significant amounts of both 2-pentenes and C6s, can be used as a feed material for this invention.
The prior art has deliberately made mixtures of 1-butene and isobutylene, see the Kelly patent cited above. In this patent the required feed material for the process is, at a minimum, a mixture of 2-pentene, 2-methylbutene-2, and C6s. This feed material is then subjected to a deep, selective hydrogenation and fractionation process to remove essentially all of the C6s, and to substantially increase (double) its 2-pentene and 2-methylbutene-2 content over that of the original feed material. The product of this hydrogenation/fractionation process contains substantial amounts of the required 2-pentene (greater than 15 wt %) and 2-methybenzene-2 (greater than 17 wt %), and no C6s. This product is then subjected to metathesis with ethylene to form a final product containing primarily 1-butene, isobutylene, and propylene. The propylene is separated as a product of the process leaving an end mixture of 1-butene and isobutylene as a co-product. The patent states that this mixture of 1-butene and isobutylene “is difficult to separate by simple fractionation due to the close proximity of their boiling points”, i.e., less than 1 degree F. The patent then suggests separation techniques such as an alcohol reaction or absorption for separating 1-butene from isobutylene which are more complicated and expensive than simple distillation. In contrast to the Kelly patent, the instant invention tolerates C6s in its feed material, does not require the separation of C6s there from, and does not make isobutylene from 2-methylbutene-2. This invention provides an end mixture containing C4s, C5s, and C6s from which high purity 1-butene can readily be separated by way of simple, low-cost distillation. This is so because no isobutylene, butadiene (particularly 1,3 butadiene), isobutane, or n-butane is present in the end mixture to be distilled, the nearest boiling compound to 1-butene in this end mixture being 2-butenes, both of whose isomers have a boiling point (b.p.) more than 10F higher than 1-butene. A boiling point separation between two compounds of more than 10 degrees F. readily allows for separation of those compounds by low-cost, simple distillation.